Organofunctionalized aerogels

ABSTRACT

The invention relates to aerogels containing functional residues of the formula (I)--Y--Z, wherein Y is a straight-chained or branched alkylene group with 1 to 22 carbon atoms; Z is halogen, pseudohalogen, SR 1 , PR 2  R 3 , colorant residue or metal complex residue; R 1  is H, a straight-chained or branched alkyl group with 1 to 22 carbon atoms or an aryl group with 4 to 10 carbon atoms; and R 2  and R 3  are identical or different, a straight-chained or branched alkyl group with 1 to 22 carbon atoms, or an aryl group with 4 to 10 carbon atoms. The aerogels are suitable, for example, as catalyst stayes and/or sensors. They can be obtained by metal alcoholates and metal alcoholates in which at least one alcoholate residue is replaced by the --Y--Z group being converted into a lyogel by hydrolysis and condensation, and subsequently being dried.

The invention relates to organofunctionalized aerogels, to a process fortheir preparation, and to their use.

Aerogels are highly porous low-density materials, prepared by forming agel and subsequently eliminating the liquid with preservation of the gelstructure.

According to a narrow definition (see e.g. Gesser and Gaswami, Chem.Rev. 1989, 89, 767) the term aerogel is understood to refer to amaterial in which the liquid has been removed from the gel undersupercritical conditions, whereas, when the gel is dried undersubcritical conditions, the resulting product is called a xerogel, andwhen the liquid is eliminated from the frozen state by sublimation, theproduct is called a cryogel.

Aerogels within the meaning of the present invention comprise all thesematerials, and may also contain any gas besides air.

Because of their high porosity, aerogels have interesting physicalproperties which make them suitable for use, among other things, as heatinsulating materials, acoustic materials, luminescent solar collectors,gas filters, catalysts or supporting materials.

For many of these applications, it is desirable to be able to modify thechemical properties of aerogels, e.g. through the incorporation offunctional groups.

DE-A 40 02 287 describes functionalized inorganic xerogels. However,under the conditions of preparation indicated therein, no productshaving a gel structure are obtained, thus the products are not aerogelswithin the meaning of the present invention.

Aerogels etherified with amino alcohols are described in U.S. Pat. No.5,270,027. However, such ether bridges do not have a particularly longstorage stability, so that a gradual splitting-off of the organic groupstakes place.

EP-A 0 629 442 discloses aerogels containing chelated transition metalsas catalysts, and Cao and Hunt (Mat. Res. Soc. Symp. Proc. 1994, Vol.346, 631) describe amino-functionalized aerogels.

Schubert et al. (Mat. Res. Soc. Symp. Proc. 1994, Vol. 348, 151) havesynthesized aerogels containing methacryloxypropyl and glycidoxypropylgroups.

However for the above-mentioned fields of application, there continuesto be a demand for additional organofunctionalized aerogels.

Surprisingly, it has now been found that aerogels having(pseudo)halogen, thio and phosphano functions on their inner surface canbe prepared without destroying these functional groups or the gelstructure during the course of the production process.

Hence the present invention relates to an aerogel which containsfunctional groups of formula (I),

    --Y--Z                                                     (I)

wherein

Y represents a straight-chain or branched alkylene group having 1 to 22,preferably 1 to 12, and, by particular preference, 2 to 3 carbon atoms;

Z represents halogen, preferably Cl, Br or I, and, by particularpreference, Cl; pseudohalogen, preferably CN or SCN, SR¹, PR² R³, a dyeresidue or a metal complex residue;

R¹ represents H, a straight-chain or branched alkyl group having 1 to22, preferably 1 to 12 carbon atoms, or an aryl group having 4 to 10carbon atoms, and preferably phenyl (Ph) or naphthyl;

R², R³ are the same of different, and preferably the same, and representa straight-chain or branched alkyl group having 1 to 22, and preferably1 to 12, carbon atoms, or an aryl group containing 4 to 10 carbon atoms,and preferably phenyl (Ph) or naphthyl.

In general, the aerogels used are those based on metal oxides suitablefor the sol-gel technology (see e.g. C. J. Brinker and G. W. Scherer,Sol-Gel Science, 1990, Chapters 2 and 3), such as Si, Al, Ti, Sn or Zrcompounds, or those based on organic substances suitable for the sol-geltechnology, such as melamine-formaldehyde condensates (U.S. Pat. No.5,086,085) or resorcinol-formaldehyde condensates (U.S. Pat. No.4,873,218). However, they can also be based on mixtures of theaforementioned materials. Used by preference are aerogels containing Sior Al compounds, particularly Si compounds; SiO₂ aerogels areparticularly preferred.

The aerogels according to the invention preferably contain thefunctional groups --Y--Z linked directly to the metal, semimetal orcarbon component of the aerogel, i.e., for example, to Si, Al, Ti, Sn,Zr or C. As a result, the organic functional group is, preferably, notlinked to the aerogel via an oxygen atom.

The aerogels of the invention can be prepared e.g. from mixtures of puremetal alcoholates, particularly of Si, Al, Zr, Ti and Sn alcoholates,and from those in which at least one, preferably an alcoholate group, isreplaced by the --Y--Z group. In that case the component containing the--Y--Z group can be added in an amount of up to 60 mole-%. Here, theterm "metal alcoholate" includes the corresponding semimetal or evencarbon compounds. Preferred are mixtures of tetraalkoxysilanes [Si(OR)₄,wherein R represents C₁ -C₁₂ -alkyl, preferably methyl or ethyl), andtrialkoxysilanes [(RO)₃ Si--Y--Z, where R represents C₁ -C₁₂ -alkyl, andY and Z have the meanings indicated above].

Both the metal alcoholates used and the organomodified metal alcoholatesare commercially available, or can be prepared by methods that are knownper se and are familiar to persons skilled in the art.

Such methods are described e.g. in the article "Hybrid Inorganic-OrganicMaterials by Sol-Gel Processing in Organofunctional Metal Alkoxides" (U.Schubert et al., Chem. Mat. 1995, 7, 2010). Methods of preparation ofthe preferred organolalkoxysilanes, such as e.g. hydrosilylation ofunsaturated compounds followed by alcoholysis; ##STR1## are described,among other places, in W. Noll, Chemie und Technologie der Silicone[Chemistry and Technology of Silicones], Verlag Chemie, Weinheim, 1968,and in U. Deschler, P. Kleinschmit and P. Panster, Angew. Chem. 1986,98, 237 (Int. Ed. Engl. 1986, 25, 236).

Numerous individual examples are given by E. P. Plueddermann, SilaneCoupling Agents, 2nd edition, pp. 31-54, Plenum Press, New York 1991.

Available commercially are e.g. alkoxysilanes (R'O)₃ --SiR, wherein Rstands for bromooctyl, bromophenyl, 3-bromopropyl, 3-butenyl,chloroethyl, chlorophenyl, 3-chloropropyl,2-(4-chlorosulfonylphenyl)ethyl, 2-cyanoethyl, 3-cyanopropyl,2-(3-cyclohexenyl)ethyl, diethylphosphanoethyl, (heptafluoroisopropoxy),iodopropyl, 3-isocanatopropyl, 3-mercaptopropyl or 3-thiocyanatopropyl.

Suitable for use as a dye is e.g. the derivative of an NLO dye, which isderived from the parent compound Disperse Red in such a way that theN(Et)CH₂ CH₂ OH group is replaced by the N(Et)CH₂ CH₂ OC(O)NH(CH₂)₃Si(OMe)₃ group. The preparation of this compound is described in thedissertation of Heinrich Stein, University of Wurzburg, 1994, and thepreparation of similar derivatives is described e.g. in F. Chaput, D.Riehl, Y. Levy, and J. P Bollot, Chem. Mater. 1993, 5, 589; F. Chaput,J. P. Bollot, D. Riehl and Y. Levy, J. Sol-Gel Sci. Technol. 1994, 2,779; B. Lebeau, C. Guermeur and C. Sanchez, Mat. Res. Soc. Symp. Proc.1994, 346, 315; M. Ueda, H. B. Kim, T. Ikeda and K. Ichimura, J.Non-Cryst. Solids 1993, 163, 125; Z. Yang, C. Xu, B. Wu, L. R. Dalton,S. Kalluri, W. H. Steier, Y. Shi and J. H. Bechtel, Chem. Mater. 1994,6, 1899.

Instead of these or other NLO dyes, it is also possible to usechromophores (functional group Z) for other uses, e.g. fluorescent dyes,pH indicators, photochromic dyes, laser dyes or dyes for aerogelcoloring. In all cases a precondition is that the actual dye molecule(=the chromophoric unit) be derivatized with a (CH₂)_(n) Si(OR)₃ unit,in order to assure, in the sol-gel process, a covalent binding of thechromophore to the aerogel structure. The advantage of this method incomparison with a mere insertion of the dye molecule into the aerogelmatrix, is that bleeding can be largely or completely prevented. Furtherexamples of usable dye derivatives are described in the article "HybridInorganic-Organic Materials by Sol-Gel Processing of OrganofunctionalMetal Alkoxides" (K. Schubert et al., Chem. Mater., 1995, 7, 2010).

It is surprising that chromophores may be incorporated in aerogels viacovalent bonds, without thereby significantly destroying the aerogelstructure, and without destroying the dye molecule under the productionconditions.

Suitable for use as metal complex is e.g. the derivative of acatalytically active metal complex, which is derived from the parentcompound Rh(CO) (Cl) (PPh₃)₂ [sic] in such a way that the PPh₃ ligandsare replaced by PPh₂ CH₂ CH₂ Si(OMe)₃. The preparation of this compoundis described by B. E. Mann, C. Masters and B. L. Shaw, J. Chem. Soc.Dalton Trans. 1972, 704.

Instead of this complex, other metal complexes derivatized in a similarmanner can also be used. Further examples of usable metal complexes aredescribed in two review articles (U. Schubert, N. Husing and A. Lorenz,Chem. Mater. 1995, 7, 2010, and U. Schubert, New J. Chem. 1994, 18,1049).

It is surprising that metal complexes can be incorporated in aerogelsvia covalent bonds, without thereby significantly destroying the aerogelstructure, and without destroying the metal complex under the productionconditions.

To prepare the aerogels according to the invention, a solution of thestarting compounds in an organic solvent, preferably in an alcohol suchas methanol, ethanol or acetone, is subjected to a sol-gel process thatis known per se, as described e.g. in J. Non-Cryst. Solids 1992, 145,85; J. Sol-Gel Sci. Technol. 1994, 2, 103, or Mat. Res. Soc. Symp. Proc.1994, 346, 151.

First, lyogels are prepared by hydrolysis and condensation, preferablyunder basic conditions, e.g. by the addition of the amount of a 0.01 Naqueous NH₄ OH solution required for the hydrolysis of the metalalcoholate groups. In so doing, the density of the subsequent aerogelsis preferably adjusted to a value of from 50 to 200 kg/m³ through theaddition of the solvent. The sol can preferably be after-stirred for ashort time, preferably for 5 minutes, to ensure thorough mixing.

After gelling, the gel is preferably aged for a relatively long time,preferably at an elevated temperature, and more preferably below theboiling point of the solvent.

Thereupon the gel is dried by known methods under supercritical orsubcritical conditions.

A process for subcritical drying is described e.g. in DE-A 43 16 540.

However, it is preferred to carry out the drying under supercriticalconditions. The critical constants for the respective solvent can beobtained from well-known reference tables, e.g. from the Handbook ofChemistry and Physics, 40th edition (1958), pages 2302 to 2304. Forexample, the critical temperature and critical pressure are, for carbondioxide, 31.1° C. and 73.0 atm.; for methanol, 240° C. and 78.7 atm.;for ethanol, 243.1° C. and 63.1 atm.; for n-propanol, 263.7° C. and49,95 atm., and for isopropanol, 235° C. and 53 atm. The drying of thegel under supercritical conditions can be carried out e.g. on the modelof EP-A-0 067 741 (=U.S. Pat. No. 4,432,956), and by the processdescribed by U. Schubert et al., J. Non-Cryst. Solids 1995, 186, 37-43.

Supercritical drying in CO₂ is particularly preferred, since thisoperation takes place under relatively sparing conditions.

Alternatively to the above-described process, a gel may be prepared alsofrom metal alcoholates, as described above, and in that case, beforedrying, the organomodified component is added to the gel e.g. before orduring ageing. To speed up the reaction of the organomodified componentwith the inner surface, heating can optionally be carried out. Acatalyst, e.g. an acid or a base, may also be used.

In that case, as described above, the drying can be carried out undersupercritical or subcritical conditions.

The aerogels according to the invention are obtained in a monolithicform. They contain the predominant part of the functional groups on theinner surface. They are frequently opaque or transparent. Surprisingly,with halogens such as Cl, and pseudohalogens such as CN, functionalizedaerogels are distinguished by a particularly high degree oftransparency.

The aerogels according to the invention are suitable for use e.g. in thefield of catalysts, e.g. for linking metal ions or metal complexes, oras sensors.

The invention will be explained in greater detail by the followingembodiments, without thereby limiting the scope of the invention.

EXAMPLES

The inner surface area of the aerogels was determined by the BET method,following the indications of DIN 66 131.

Example 1

20.55 g (135 mmoles) of tetramethoxysilane and 2.97 g (15 mmoles) ofcommercially available chloropropyltrimethoxysilane are treated with13.27 g (414 mmoles) of methanol. For the hydrolysis and condensation,10.53 g (585 mmoles) of a 0.01 N NH₄ OH solution is added to thissolution, and the mixture is stirred for 5 minutes. After 30 minutes themixture is in the form of a lyogel. After aging for 7 days in the closedvessel at 30° C. the lyogel is transferred to an autoclave and driedwith CO₂ under supercritical conditions (with cooling, 4-day rinsingprocess: Flow rate 200 mL/min, heating to 40° C. at a rate of 0.25°C./min, pressure up to 100 bar, discharge of the fluid over one day).

At the end of the supercritical drying a transparent aerogel having adensity of 251 kg/m³ and an inner surface area of 875 m² /g is obtained.

Example 2

20.55 g (135 mmoles) of tetramethoxysilane and 2.95 g (15 mmoles) ofmercaptopropyltrimethoxysilane (obtainable according to U.S. Pat. No.4,082,790) in 13.21 g (412 mmoles) of methanol are prepared and treatedwith 10.53 g (585 mmoles) of a 0.01 N NH₄ OH solution. After 25 minutesa lyogel is obtained, which, after aging for 7 days at 30° C., istransferred in a closed vessel to an autoclave and dried by means of CO₂under supercritical conditions, as in Example 1.

In this way and aerogel having a density of 265 kg/m³ and an innersurface area of 654 m² /g is obtained.

Example 3

16.89 g (111 mmoles) of tetramethoxysilane and 4.06 g (12 mmoles) ofdiphenylphosphinoethyltrimethoxysilane (obtainable according toNiebergall, Makromol. Chem. 1962, 52, 218) are placed in 16.53 g (516mmoles) of methanol, and treated with 8.64 g (480 mmoles) of a 0.01 NNH₄ OH solution. After 15 minutes a lyogel is obtained which, afteraging for 7 days as described in Example 1, is dried under supercriticalconditions. A white aerogel having a density of 238 kg/m² and an innersurface area of 624 m² /g is obtained.

Example 4

12.18 g (80 mmoles) of tetramethoxysilane and 0.24 g (0.42 mmole) of thedye ##STR2## are dissolved in 25.59 g (799 mmoles) of methanol, andtreated with 5.76 g (320 mmoles) of a 0.01 N NH₄ OH solution. After 6 to7 hours a lyogel is obtained, which, after aging for 7 days as describedin Example 1, is dried under supercritical conditions. In this way adark-red aerogel having a density of 156 kg/m³ is obtained.

Example 5

25.11 g (165 mmoles) of tetramethoxysilane and 0.36 g (0.41 mmoles) ofthe metal complex trans-Rh(CO) (Cl) [PPh₂ CH₂ CH₂ SI(OMe)₃ ]₂ [sic] aredissolved in 10.83 g (338 mmoles) of methanol and treated with 11.93 g(663 mmoles) of a 0.01 N NH₄ OH solution. After 40 minutes a lyogel isobtained, which, after aging for 7 days as described in Example 1, isdried under supercritical conditions. In this way a pale yellow aerogelhaving a density of 304 kg/m³ is obtained.

What is claimed is:
 1. An aerogel, containing functional groups offormula (I)

    --Y--Z                                                     (I)

wherein the functional groups --Y--Z are bound directly to siliconatoms, and Y represents a straight-chain or branched alkylene grouphaving 1 to 22 carbon atoms; Z represents halogen, pseudohalogen, SR¹,PR² R³, a dye residue or a metal complex residue; R¹ represents H, astraight-chain or branched alkyl group having 1 to 22 carbon atoms or anaryl group having 4 to 10 carbon atoms; and R² and R³ are the same ordifferent and represent a straight-chain or branched alkyl group having1 to 22 carbon atoms or an aryl group having 4 to 10 carbon atoms.
 2. Anaerogel according to claim 1, characterized in that it is an SiO₂aerogel.
 3. An aerogel according to claim 1, characterized in that thesymbols in Formula (I) have the following meanings:Y is a straight-chainor branched alkylene group having 1 to 12 carbon atoms; Z is Cl, Br orI, CN, SCN, SR¹ or PR² R³ ; R¹ is H, a straight-chain or branched alkylgroup having 1 to 12 carbon atoms, phenyl or napthyl; and R² and R³ arethe same or different and represent a straight-chain or branched alkylgroup having 1 to 12 carbon atoms, phenyl or napthyl.
 4. An aerogelaccording to claim 1, characterized in that the symbols in Formula (I)have the following meanings:Y is a straight-chain alkylene group having2 to 3 carbon atoms; and Z is Cl, SH or --PPh₂.
 5. An aerogel accordingto claim 1, characterized in that it contains the --Y--Z group in anamount of up to 60 mole-% calculated on the silicon atoms.
 6. A processfor the preparation of organofunctionalized aerogels according to claim1, characterized in that pure metal alcoholates and metal alcoholates inwhich at least one alcoholate group is replaced by the --Y--Z group, areconverted by hydrolysis and condensation into a lyogel, and then dried.7. A process according to claim 6, characterized in that the componentcontaining the --Y--Z group is added in an amount of up to 60 mole-%. 8.A process according to claim 6, characterized in that tetraalkoxysilanesof formula Si(OR³)₄ and trialkoxysilanes of formula (R⁴ O)₃ Si--Y--Z,where R³ and R⁴ are C₁ -C₁₂ alkyl groups and the other symbols have thesame meanings as in formula (I), are converted to an organomodified SiO₂aerogel.
 9. A process according to claim 6, characterized in that thelyogel is aged before drying.
 10. A process according to claim 6,characterized in that the drying is carried out under supercriticalconditions.
 11. A process according to claim 10, wherein thesupercritical drying is carried out in CO₂.
 12. In a process forproducing a catalyst, the improvement which comprises adding anorganofunctionalized aerogel according to claim 1 as a precursor.
 13. Ina method for detecting compounds, the improvement which comprisesutilizing a organofunctionalized aerogel according to claim 1 as asensor.
 14. A sensor for chemicals which comprises an aerogel accordingto claim 1.